1. Field of the Invention
An object of the present invention is a stabilized power supply, with reduced ripple factor, which can be used especially in medicine to provide electrical power to X-ray tubes. However, it can be also applied in other fields where there is a need for high levels of electrical power, for example of up to 100 kilowatts (KW) and very high voltages, for example of up to 100 kilovolts (KV). The invention relates mainly to dc-dc type voltage elevation power supplies provided with an dc-ac converter.
2. Description of the Prior Art
A voltage rise power supply of the dc/dc type generally has a first rectifier which can be connected with an electrical mains supply. The rectified and filtered voltage given by this rectifier is introduced into a variable frequency dc-ac converter which gives a converted signal. The converted signal is then applied to a voltage-raising transformer which is itself connected with a second rectifier to produce a rectified high voltage. A rectified high voltage of this type can be used, for example, to control an X-ray tube by giving the necessary potential difference between the cathode and the anode of this tube. The high voltage given is generally controlled by causing the ripple frequency of the dc-ac converter to vary so as to affect its power transfer function according to the frequency. Despite all the precautions that may be taken, especially in choosing high capacitance filtering cells in the second rectifier or in choosing a high ripple frequency for the dc-ac converter, the persistence of ripples in the high voltage delivered is noted during use. The frequency of these ripples follows the frequency of the dc-ac converter. Besides, these ripples in the high voltage delivered are all the more sensitive as the power capable of being consumed by the tube is great. The problem therefore arises more especially with high power applications.
A problem of this type arises, for example, when there is a dc/dc power supply of this type working at a given power, for example 50 KW, and it is sought to put a new power supply into industrial use which is twice or three times or even n times the power of this existing supply. The method used lies in developing a new power supply wherein the characteristics of all the components would be doubled, tripled or multiplied by n but this method is not practicable in industrial conditions because the dispersal of the characteristics of the electronic components used is such that numerous problems arise in creating the system. The result of these problems is that this approach, which is simple in principle, cannot easily provide a solution. Lengthy, expensive research and design work is needed to achieve this goal.
Furthermore, the use of dc/ac converters in voltage-raising power supplies of the dc/dc type does have unwanted effects on the electrical lines. In particular, the reinjection, into the mains system, of reactive energy due to commutation in the dc-ac converter is quite simply doubled, tripled or multiplied by n when the power thus produced is doubled, tripled or multiplied by n times. To prevent this reinjection, it becomes necessary to place very powerful, hence very bulky and very expensive filters in the filtering circuit of the first rectifier. The cost of these filters increases with the power used.
It is an object of the invention to cope with these disadvantages by proposing a method which, at multiple power values, uses exactly those already tested industrial methods wherein the ripple factor of the high voltage at the terminals of the load, and hence the reactive energy reinserted into the mains, is considerably reduced. In practice, a reduction in a ratio of 4 is obtained. The principle of the invention lies in the use of two, three or n identical dc/ac converters parallel connected to the output of the first rectifier, the triggering of said dc/ac converters being phase shifted with respect to one another, in phase quadrature, with a phase shift of 2 .pi./3 or with a phase shift of 2 .pi./n. It can then be shown that each dc-ac converter can provide a fraction P/n of the total power P to be transmitted.
It is then easier to control the characteristics of the usable passive and active components, taking into account the operating frequencies, if these characteristics are not already known. In particular, it becomes possible, with smaller components of this type, to work at ripple frequencies which are themselves higher. Thus, in a preferred example, using two double alternation dc/ac converters, phase-shifted in quadrature, a ripple equivalent to a four-peak rectification (the ripple frequency on the load is four times the frequency of the dc-ac converter) is obtained at the terminals of the X-ray tubes. Thus, for a natural operating frequency of a given dc-ac converter, we get a high voltage ripple reduced by at least half as compared with a generator comprising a single dc-ac converter working at the same natural frequency. Furthermore, in this type of operation, the reactive current that flows through the filter of the first rectifier, is reduced in a ratio of 4 for the rated power of the instrument. Here again, a considerable gain is obtained in the sizing of the filtering capacitances.
The structure of a high voltage unit should further provide perfect balancing between a positive channel and a negative channel. Since it is unlikely that, despite all the precautions taken, both the dc/ac converters or all the small n dc/ac converters thus put into operation have equal transfer characteristics, in one improvement, the contribution of each of these dc/ac converters to the positive channel on the one hand and to the negative channel on the other hand is shared. In this improvement, the connection of the voltage raising circuit of the dc-ac converter circuit output voltage is then slightly more complicated to enable, through the use of already tested voltage raising transformers, a balancing between the channels. This balancing is independent of the characteristics of the dc/ac converters and, for each dc-ac converter, when they are double alternation dc/ac converters between one alternation and another.